The principal of image element antennas, also known as partially reflecting sheet arrays, is well known in the art. These antennas are constructed by placing a driven antenna element in a space between two parallel planes. One of the planes is totally reflecting at the antenna frequency and the other is partially reflecting. The multiple reflections between the two planes of the energy radiated by the element, with partial transmission at the partially reflecting plane, produce a radiation pattern directing most of the radiated power in a predetermined direction.
Image element antennas are taught by Giswalt Von Trentini in an article entitled "Partially Reflecting Sheet Arrays" in IRE Transactions on Antennas and Propagation, October 1956, at page 666 and by U.S. Pat. No. 3,990,078, Image Element Antenna Array, For a Monopulse Tracking System For a Missile, Jan. 6, 1975.
Image antennas depend for the increased directivity on destructive interference between the successive rays transmitted by the partially reflecting plane. If, for some reason, the planes do not produce multiple reflections for certain rays emitted by the element, the directivity of the antenna will be decreased.
As is well known, the reflected fraction of the energy incident on a dielectric is a function of the angle of incidence. Specifically, the reflected fraction has a minimum at an angle of incidence known as the Brewster angle. So an image antenna will have greatly increased sidelobes in the radiation pattern centered at the Brewster angle of the dielectric partially reflecting plane. Since one of the purposes of using image antennas is to achieve high directivity, this is a serious limitation.